Do you want an event per testcase? If no, the solution is hard because you don't know how many testcases there are to build a KV filter to work on.

Generally, this is the workflow or compute flow:
Use the split filter on the [testcases] field - this will yield X new events with the same structure.
Use the KV filter on the testcase][description1] field - delimiter => " - " separator => ":" with the target [testcase]

Something like this (I reduced the xml to make the output easier to read)...